The use of conversion layers for increasing the protective effect of cathodic corrosion protection systems and as a primer coating for lacquers and paints is known since long time. Especially on zinc, cadmium and aluminium-containing substrates, the method of chromatizing surfaces is established besides phosphatization methods.
Hereby, the surface to be treated is exposed to a treatment solution whose essential component are chromium(VI) compounds. The conversion layer produced thus also contains chromium(VI) ions. Chromatization layers usually exhibit good corrosion protection and good decorative properties. The toxicological properties of chromium(VI) are disadvantageous in the application of chromium(VI)-containing solutions and, respectively, chromium(VI)-containing coatings. The use of chromium(VI)-containing conversion layers is, thus, strongly limited, for example by EC directive 2000/53/EC (EC directive on end-of-live vehicles).
As an alternative to chromatization solutions, chromium(III)-containing, acidic treatment solutions have been proposed which are, in contrast to chromatizations, generally referred to as “passivations” and, respectively, “passivation solutions”. Said treatment solutions consist, for example as proposed in DE 196 15 664 A1, essentially of a chromium(III) salt in mineral acid solution, a dicarboxylic acid or a hydroxycarboxylic acid and a cobalt salt. Such methods, known as “thick film passivation”, are applied at elevated temperature, about 40-60° C., so as to obtain a passivation layer thickness on zinc surfaces which is sufficient for corrosion protection. The necessity to apply the method at a temperature elevated as compared to room temperature results from the very low reactivity which is characteristic for the chromium(III) ion in contrast to the chromium(VI) ion. A substantial prolongation of the reaction times can, as an alternative to increasing temperature, usually not be realized for economic reasons.
In the case of zinc alloy surfaces such as zinc-iron or, respectively, zinc-nickel or zinc-cobalt, the metal alloyed with zinc is suitable as a black pigment which may be readily produced. By treatment in an acidic solution, the less noble zinc is dissolved from the layer and the alloy metal is, finely dispersed, enriched on the surface. Hereby, the surface is coloured dark or, respectively, almost black. Such a method is, for example, described in DE 199 05 134 A1. Here, an oxidizing agent is additionally applied for the black passivation of zinc-nickel surfaces so as to facilitate the etching effect of the acid. The result is a black surface which, however, does not offer significant corrosion protection.
According to U.S. Pat. No. 5,415,702, Cr(VI)-free black conversion layers on zinc-nickel alloy layers may, alternatively, be produced by treatment with acidic, chromium(III)-containing solutions which further contain oxygen acids of phosphorus. In said method, homogeneously black conversion layers having good decorative properties are formed. In laboratory experimentations, we could, however, not reproduce the corrosion protection described in said document.
WO 03/05429 describes a similar conversion layer which is as well produced using a chromium(III)-containing, acidic treatment solution which further contains phosphate ions. Said surface exhibits as well good decorative properties but does, however, not yield sufficient corrosion protection properties without further steps of post-treatment such as top-coating (“Versiegelung”).
EP 1 484 432 A1 describes chromium(III)-containing black passivation solutions for zinc alloy surfaces containing chromium(III) ions and nitrate as well as carboxylic acids, such as tartaric acid, maleic acid, oxalic acid, succinic acid, citric acid, malonic acid or adipic acid. The surface treated therewith must be subjected to a subsequent top-coating so as to improve the corrosion protection. The treatment solutions are applied at temperatures above normal room temperature.
US 2004/0156999 describes as well a method for the black passivation of zinc alloy surfaces. The treatment solutions contain, besides chromium(III) ions and phosphorus-containing anions, nitrate and an organic carboxylic acid. Examples mentioned for the organic carboxylic acids are citric acid, tartaric acid, maleic acid, glyceric acid, lactic acid, glycolic acid, malonic acid, succinic acid, oxalic acid and glutaric acid. Using the treatment solutions described, we did not succeed in realizing the corrosion protection described therein.
Accordingly, it is not possible to produce black passivated zinc and, respectively, zinc alloy surfaces in a fully satisfying manner using the known methods. A particular disadvantage of the described methods is that a black zinc alloy surface providing a good basic corrosion protection (“Grundkorrosionsschutz”) cannot be successfully produced. Consequently, post-treatment steps are basically required for improving the corrosion protective properties of the layer.